Abstract

The circadian locomotor rhythm of the cricketGryllus bimaculatus is primarily generated by a pair of optic lobe circadian pacemakers. The two pacemakers mutually interact to keep a stable temporal structure in the locomotor activity. The interaction has two principal effects on the activity rhythm, i.e., phase-dependent modulation of the freerunning period and phase-dependent suppression of activity driven by the partner pacemaker. Both effects were mediated by neural pathways, since they were immediately abolished after the optic stalk connecting the optic medulla to the lobula was unilaterally severed. The neural pathways were examined by recording locomotor activity, under a 13 h light to 13 h dark cycle, after the optic nerves were unilaterally severed and the contralateral optic stalk was partially destroyed near the lobula. When the dorsal half of the optic stalk was severed, locomotor rhythm mostly split into two components: one was readily entrained to the given light-dark cycle and the other freeran with a marked fluctuation in freerunning period, where the period of the freerunning component was lengthened or shortened when the onset of the entrained component occurred during its subjective night or day, respectively. The phase-dependent modulation of activity was also observed in both components. However, severance of the ventral half of the optic stalk resulted in appearance only of the freerunning component; neither the phase-dependent modulation of its freerunning period nor the change in activity level was observed. These results suggest that neurons driving the mutual interaction and the overt activity rhythm run in the ventral half of the proximal optic stalk that includes axons of large medulla neurons projecting to the cerebral lobe and the contralateral medulla.